If you've never heel and toe'd before, it might be best to even start with the car off and parked, just to initially get a feel of where your foot will go on the pedals since it will likely feel unnatural initially. Since you're not moving, you can look to see where your foot is best positioned. Then start the engine, and practice the feel of blipping the throttle while still parked. You don't want to be moving with the distraction of trying to figure out where your foot should go/move.